takes over as the active switch. If the VSL goes down, one core switch knows the status of the other and does
not change its state.
PAgP Interaction with Other Features
The Dynamic Trunking Protocol (DTP) and the Cisco Discovery Protocol (CDP) send and receive packets
over the physical ports in the EtherChannel. Trunk ports send and receive PAgP protocol data units (PDUs)
on the lowest numbered VLAN.
In Layer 2 EtherChannels, the first port in the channel that comes up provides its MAC address to the
EtherChannel. If this port is removed from the bundle, one of the remaining ports in the bundle provides its
MAC address to the EtherChannel. For Layer 3 EtherChannels, the MAC address is allocated by the active
switch as soon as the interface is created (through the interface port-channel global configuration command).
PAgP sends and receives PAgP PDUs only from ports that are up and have PAgP enabled for the auto or
desirable mode.
Link Aggregation Control Protocol
The LACP is defined in IEEE 802.3ad and enables Cisco switches to manage Ethernet channels between
switches that conform to the IEEE 802.3ad protocol. LACP facilitates the automatic creation of EtherChannels
by exchanging LACP packets between Ethernet ports.
By using LACP, the switch or switch stack learns the identity of partners capable of supporting LACP and
the capabilities of each port. It then dynamically groups similarly configured ports into a single logical link
(channel or aggregate port). Similarly configured ports are grouped based on hardware, administrative, and
port parameter constraints. For example, LACP groups the ports with the same speed, duplex mode, native
VLAN, VLAN range, and trunking status and type. After grouping the links into an EtherChannel, LACP
adds the group to the spanning tree as a single switch port.
LACP Modes
LACP modes specify whether a port can send LACP packets or only receive LACP packets.
Table 16: EtherChannel LACP Modes
DescriptionMode
Places a port into an active negotiating state in which the port starts negotiations with
other ports by sending LACP packets.
active
Places a port into a passive negotiating state in which the port responds to LACP packets
that it receives, but does not start LACP packet negotiation. This setting minimizes the
transmission of LACP packets.
passive
Both the active and passive LACP modes enable ports to negotiate with partner ports to an EtherChannel
based on criteria such as port speed, and for Layer 2 EtherChannels, based on trunk state and VLAN numbers.
Ports can form an EtherChannel when they are in different LACP modes as long as the modes are compatible.
For example:
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
114 OL-29424-01
Configuring EtherChannels
Link Aggregation Control Protocol
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